High voltage power cables have multiple coatings that are extruded onto the conductor to provide protection and to extend the life of the power cable. In particular, high voltage power cables have an inner semiconducting layer (referred to as conductor shield) surrounding the conductor, an intermediate layer of cross-linked polyethylene insulation and an outer semiconducting layer surrounding the insulation layer (referred as insulation shield). The purpose of the inner semiconducting layer is to relax or relieve the heterogeneous electrical stress attributed to irregularities in the conductor and also to increase the adhesion between the conductor and insulation. The external semiconducting layer, which could be either a bonded layer or a strippable layer, homogenizes the electrical stress on the insulation surface.
The semiconducting layer, which is also known as the conductor shield, is typically formed from ethylene based polymers e.g. ethylene/vinyl acetate or ethylene/ethyl (or butyl) acrylate or ethylene/alkene copolymers. The ethylene based polymer is blended with conducting carbon black e.g. furnace black or acetylene black, appropriate antioxidants and an organic peroxide cross-linking agent.
The level or amount of the trace elements (impurities) in the carbon black, conductor shield asperities at the insulation interface and the additives in the composition all tend to influence the useful life of the insulation.
A common cause of reduction in the life of an extruded power cable is the formation of so-called water trees, which are believed to result from the presence of water and water-soluble ions, such as sulphur and metallic cations, within the conductor shield. Carbon blacks with low sulphur and cation impurity levels are advantageous for use in long-life underground cables. Acetylene black manufactured from pure acetylene is considered to be the industry standard for a clean carbon black.
The asperities at the conductor shield--insulation interface tend to increase localized electrical stress on the insulation and hence reduce the life of the cable. The actual size and number density of these asperities can be reduced so as to increase the cable life, by choosing appropriate manufacturing conditions or carbon black of appropriate morphology and cleanliness characteristics, or both. U.S. Pat. No. 5,352,289 describes a furnace carbon black having an ash level and sulphur content less than or equal to 50 ppm as being suitable for cable applications. However, it is also known that, under optimum processing conditions, furnace blacks of low ash and grit content exhibited a smoothness quality comparable to acetylene black.
Some of the low molecular additives conventionally added to highly-filled conductor shield compositions have also been found to be advantageous in increasing or extending the performance of extruded power cables. For instance, U.S. Pat. No. 4,909,960 describes one such composition containing a low molecular weight polyethylene with an average molecular weight of 1000-4000. U.S. Pat. No. 4,612,139 describes the use of polyethylene glycol having molecular weight of 1000-20,000 as imparting advantages in retarding water tree growth. U.S. Pat. No. 4,801,766 describes use of a conductor shield composition containing n-vinyl carbazole to increase the breakdown strength of the cable.
U.S. Pat. No. 5,719,218 discloses addition of an ethylene/vinyl acetate(vinyl alcohol) ter-polymer to the intermediate layer of insulation, to improve resistance to moisture induced degradation.
Other methods to impart to the insulation an intrinsic resistance to the growth of water trees are known. However, there remains a need for an improved conductor shield composition which, when laid (layered) with a conventional non-tree retardant insulation, will extend cable life.